· That you will you will have more than one copy of an Exchange DB in a DAG. OK, you are allowed a duh!

· That you will be deploying Exchange 2010 DAG with cheap JBODs, which will undoubtedly save you a bundle.

· If you are going to use JBOD, then you really understand how to interpret MTBF and its relationship to uptime SLA.

All Exchange Mailbox Servers in one site (HA but no DR)

If you want to deploy Exchange 2010 with JBOD, it is recommended that you should deploy with at least three copies. In addition to this, if you want to also use the DAG copies for Point In Time (PIT) recovery, then you will need a server to host the lagged copy of the DB. Lagged copies are a means to safeguard against store/logical corruption events (and possibly accidental mailbox deletions). If you are going to distribute the lagged copies among your primary severs, then you will need at least two lagged copies. If you are going to use dedicated servers with lagged copies with JBOD, for better reliability you are well advised to deploy enough servers to house two lagged copies; otherwise you could deploy dedicated lagged copy servers with RAID and thus only have one lagged copy per database.

So how many copies are we up to? …and how many servers?

OK, three copies per DB; two more for PIT lagged copies; one less if you share and distribute the lagged copy on more than one primary server or if you use RAID for PIT. That is, 1, 2, 3, wait. All right, you are big boys/girls (smart Admin people), you know how to fire up Microsoft Excel to do the math and cost it out.

From all accounts, recovering PIT from lagged copies is not a walk in the park. In fact, you are well advised not to plan for it too frequently. Plus, you can only have a maximum of 14 day replay lag for lagged copies. This is a limit imposed by Exchange 2010. If you also decide to share the primary servers and implement lagged copies, then you will be using the primary mailbox server for your recoveries – so plan your IOPS so that you do not impact Outlook client SLAs during these recoveries and re-seeds and manage your Active and Passive nodes carefully.

Introduce DPM into this equation:

Let us say you implement only three copies of an Exchange DB with JBOD, and use DPM for backup and PIT recovery. Now what? You have: